This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The class I ligase ribozyme promotes the formation of a 3'-5'phosphodiester linkage between a small oligonucleotide substrate and its own 5'terminus, requiring both the nucleophile and electrophile to be situated within the context of a Watson-Crick pair. This chemistry [unreadable]among the fastest yet observed for any catalytic RNA [unreadable]is equivalent to a single turnover of template-directed RNA polymerization, an activity that would have been requisite in the RNA World. Indeed, minor variants of the ligase are capable of catalyzing high-fidelity template-directed RNA polymerization using exogenous nucleoside triphosphates, and the ligase constitutes the catalytic core for a general RNA polymerase ribozyme. To understand the basis of RNA-catalyzed RNA-dependent RNA polymerization, we have set out to solve the crystal structure of the ligase.